1. Field of the Invention
This invention relates to photopolymerizable compositions which are useful as coating and printing ink vehicles and more particularly to pigmented photopolymerizable coatings and inks which are stable to premature gelation.
2. The Prior Art
Printing inks and coatings prepared using photopolymerizable vehicles are known to the art which can be polymerized and dried by exposure to ultraviolet radiation as example, U.S. Pat. No. 2,453,769, 2,453,770, 3,013,895, 3,051,591, 3,326,710, 3,511,710, 3,772,062 and 3,804,735.
These photopolymerizable inks and coatings are generally pigmented ethylenically unsaturated polyester compositions. When photoinitiators are incorporated in the composition, thin liquid films of the photopolymerizable composition can be cured to a hard film within 1 second or less using medium pressure mercury lamps.
The use of photopolymerizable inks and coatings eliminates the use of volatile organic solvents and the high cost and time of oven-baking which has heretofore been the conventional practice for drying and hardening inks and coatings.
In formulating inks using photopolymerizable unsaturated polyester vehicles, it was found that for some unexplainable reason, when the ester vehicles, which are normally stable to gelation at room temperature were pigmented with colorants, the ester vehicle self-polymerized within a short time to form undesirable gels. The formation of gels in the ink or coating formulation substantially destroys the commercial utility of the formulation. Preventing the gelation of the pigmented formulation or at least extending the time in which a pigmented composition will gel is important in commercial practice as several weeks storage is often required between formulation of the ink or pigmented coating and actual use of the formulation in printing and coating applications.